This invention relates to printing machines, and more particularly, to a development and/or cleaning apparatus for developing latent electrostatic images on a charge-retentive surface or removing residual developer therefrom and, more particularly, to an improved magnetic roll structure for presenting single component magnetic developer to a charge-retentive surface such as a photoconductor or removing residual developer therefrom.
In the art of xerography or other similar image reproducing arts, a latent electrostatic image is formed on a charge-retentive surface such as a photoconductor which generally comprises a photoconductive insulating material adhered to a conductive backing. The photoconductor is first provided with a uniform charge after which it is exposed to a light image of an original document to be reproduced. The latent electrostatic images, thus formed, are rendered visible by applying any one of numerous pigmented resins specifically designed for this purpose. In the case of a reusable photoconductive surface, the pigmented resin, more commonly referred to as developer or toner which forms the visible images is transferred to plain paper.
Subsequent to the transfer of the developed image the charge-retentive surface is prepared for further use by removing the residual developer therefrom. This can be accomplished by the same structure that is used for the development of the latent image.
It should be understood that for the purposes of the present invention, which relates to the development of latent electrostatic images with developer particles or removing residual developer particles from a charge-retentive surface subsequent to transfer, the latent electrostatic image may be formed by means other than by the exposure of an electrostatically charged photosensitive member to a light image of an original document. For example, the latent electrostatic image may be generated from information electronically stored or generated in digital form which may afterwards be converted to alphanumeric images by image generation electronics and optics. However, such image generation electronic and optic devices form no part of the present invention.
One general approach to developing electrostatic images, which is often used commercially, is to attract particulate developer to an applicator surface from the outlet of a developer sump or housing and move the applicator into a transfer relation with the imaging member so that the particles can adhere to the member in accordance with the image pattern. Most commonly, the applicator is a roller which rotates so that its peripheral surface moves between the sump outlet and a zone in transfer relation with the imaging member. Adherence of the developer to such applicator rollers can be accomplished in various ways including, e.g., adhesive or electrical attraction, but the most prevalent commercial technique utilizes magnetic attraction and applicators using this technique are often called magnetic brushes. The same structure can be employed for removing residual developer from a charge-retentive surface.
Developers used with such magnetic brushes can be single component or they may comprise a mixture of two components in which case the toner particles are electrostatically attracted to magnetically attractable carrier particles in the developer mixture. The magnetic brush applicators can take various forms, however, a typical configuration comprises a non-magnetic outer cylinder which surrounds an array of magnets located within its inner periphery. Developer movement is effected by rotation of the outer cylinder and/or the interior magnet array.
Although single component developers offer many advantages over two component systems it is necessary to magnetically load them in order to be able to effect proper development of electrostatic images with a magnetic roll or to satisfactorily remove residual developer from the charge-retentive surface subsequent to image transfer. In the case where low magnetic loading is desired conventional magnetic roll structures do not produce satisfactory results.
Conventional magnetic roll structures must be placed very close to the charge-retentive surface because the magnetic field falls off very rapidly with distance. The spacing tolerance is quite critical with conventional structures.
As will be appreciated, a single component magnetic developer transport which can attract such developers even when they contain small quantities of magnetic material is desirable.
As will be further appreciated a single component magnetic developer transport which is less sensitive to spacing tolerances is also desirable.